When an internal combustion engine is operated, a requested torque is calculated in accordance with an accelerator operation amount. Further, an intake air amount necessary for generating the requested torque is determined. Subsequently, for example, a throttle valve opening (throttle opening), ignition timing, and fuel injection amount are selected in accordance with the intake air amount. Due to the volumetric capacity of an intake path positioned downstream of a throttle valve, however, there is a response lag (phase lag) between the instant at which the throttle opening is changed to the selected throttle opening and the instant at which an amount of air according to the selected throttle opening is actually supplied into a cylinder. Therefore, the selected ignition timing and fuel injection amount may not always be appropriate for the actual intake air amount.
Meanwhile, a control device disclosed in JP-A-1991-182667 controls the ignition timing and fuel injection amount during a transient operation of an internal combustion engine in accordance with the amount of intake air actually taken into a cylinder. More specifically, to compensate for a phase lag of the intake air amount, this control device estimates the amount of air to be actually supplied into the cylinder before the determination of the ignition timing and fuel injection amount and calculates an actual torque, which is to be actually generated, in accordance with the estimated air amount. Subsequently, this control device determines an optimum fuel injection amount and optimum ignition timing in accordance with the actual torque. The control device described above can compensate for a phase lag of the intake air amount even during a transient operation, supply the right amounts of fuel and air to each cylinder, and achieve ignition with optimum timing.
However, the control device determines the optimum fuel injection amount and ignition timing in accordance with the actual torque after estimating the actual intake air amount whose phase lag is to be compensated for and determining the actual torque from the actual intake air amount. Therefore, this may increase control parameter computation load, thereby causing control lag. The control process described above determines the optimum ignition timing and fuel injection amount in accordance with the calculated actual torque, but does not compensate for a response lag between the requested torque and the actual torque, which is based on the phase lag of the intake air amount. Therefore, it is difficult for the control device to change the intake air amount in accordance with the requested torque and generate torque in accordance with the requested torque in a situation where the requested torque changes within a period shorter than the phase lag time of intake air while an internal combustion engine is conducting, for instance, a transient operation.